In general, as a continuous manufacturing process of long metal products, forming rolls are ordinarily used. A typical process among them represented by an electric-resistance-welded pipe process ordinarily includes a pre-process for rewinding a metal coil as a material to supply the metal coil to a forming process, an initial forming process carried out by break down rolls, cluster rolls and fin pass rolls, a welding process for welding opposed edge parts of a strip material by, for instance, a high frequency welding, a sizing process for correcting the roundness and the straightness of the pipe by correction rolls and a cutting process for cutting the manufactured metal pipe to a prescribed length.
A forming method of the long metal products generally classified the above-described roll forming method and a press forming method. In the press forming, a material to be formed basically receives only a two-dimensional deformation in a section, so that the material to be formed has less excessive distortion and residual stress, and a dimensional accuracy of the product is easily obtained. However, an investment for plants and equipments including a metal die is high, productivity is low and the length of the product is restricted. On the other hand, in the roll forming, since not only the investment for plants and equipments is low, but also a continuous production can be realized, the productivity is also high. Further, the length of the product is scarcely restricted. However, since the material to be formed receives three-dimensional deformation depending on forming rolls, below-described disadvantages arise.
Most of fundamental problems in the roll forming reside in that a forming tool is rolls as a rotating member and a radius of rotation thereof cannot be made very large due to the restriction of a production capacity, a cost or the like. Therefore, specifically, problems as stated below arise.
(1) A feature of the three-dimensional deformation such as a winding of the material round the rolls is strong. Not only a deformation in a section as an object of forming, but also various additional deformations and distortion in other directions are generated. As a result, a total distortion is large and a state of the residual stress is complicated, which gives a bad influence to the dimensional accuracy and an intrinsic quality of the product.
(2) Since a peripheral speed difference in a contact area between the rolls and the material to be formed is large, a problem frequently arises in view of the surface quality of the product due to a relative slip between both the rolls and the material to be formed.
(3) Since the contact area between the rolls and the material to be formed is small in comparison with a severe deformation, a surface pressure between the rolls and the material be formed is high. As a combined effect of the high surface pressure and the peripheral speed difference, the wearing of the rolls is serious and a cost for maintaining the dimensional accuracy of the product becomes high.
(4) As an insertion resistance which the material to be formed receives from the roll is large, It is frequently caused insufficient thrust force, and necessary driving energy becomes high.
For instance, in the sizing process of the above-mentioned continuous manufacturing process of the metal pipe, two-way rolls stand, three-way rolls stand or four-way rolls stand is used which is arranged on the same plane in which central axes are vertical to an axis of the pipe. In any of combinations of these rolls, such a pass is formed as to hold substantially all the periphery of an outer surface of a material pipe.
As a structure that obtains a high reduction ratio per one pass for the purpose of reducing the number of forming roll stands in the sizing section, a method is proposed that outside diameters of the opposed right and left rolls of the four-way rolls stand are made to be smaller than outside diameters of the opposed top and bottom rolls, and the opposed right and left rolls are arranged at positions more upstream than positions of the opposed top and bottom rolls (PTL 1).